The present invention relates to a device for controlling the pressure in the auxiliary-pressure supply system of a vehicle hydraulic brake unit, which device includes a diaphragm-type or bladder-type pressure accumulator, wherein the operating pressure is maintained by a hydraulic pump driven by a motor adapted to be switched on and off in response to pressure.
Brake units comprising a hydraulic pressure accumulator which is loaded by means of an electromotively driven hydraulic pump are known. In these units, attainment of a predetermined maximum operating pressure, a pressure which electrically disconnects and de-activates the electric motor and hydraulic pump until the working pressure has fallen to a minumum threshold value.
Diaphragm-type accumulators or bladder-type accumulators typically are filled with gas, most often nitrogen which is compressed by the hydraulic pressure developed by the pump. The hydraulic medium takes up the volume of gas displaced. A rubber-elastic diaphragm subdivides the interior of the accumulator into two chambers, one of which contains the gas, and the other the hydraulic medium.
Since for various reasons it is impossible to refill gas into these type pressure accumulators of a brake unit, the useful life of such accumulators is to a substantial degree dependent on the preservation of the gas inflation pressure. The permeation of gas through a rubber-elastic diaphragm as typically used in pressure accumulators cannot be prevented entirely and steps aimed at reducing the permeation of gas to a minimum are desirable. Heretofore, it has been proposed to use a special mixture of nitrogen and a gas having a large molecular diameter, such as sulfurhexafluoride as disclosed in the German patent application No. P 33 13 910.5. Compared to a gas of pure nitrogen, which is particularly adapted for use in pressure accumulators, the permeation is reduced considerably due to the added sulfurhexafluoride, and in particular at the high temperatures prevailing in the engine compartment of an automotive vehicle. Certain unfavorable properties of sulfurhexafluoride are of comparatively minor importance in comparison to usefulness of achieving its intended purpose.
Furthermore, it is known from the German printed and published patent application No. 29 10 554 to interpose a third compartment, namely a closed fluid compartment, between the compressible gas filled chamber and the hydraulic chamber. In this arrangement, the gas compartment is isolated from the interposed third compartment by a metal wall designed as a bellows which is practically gas-impermeable, while a rubber-elastic diaphragm which performs the control functions isolates the two fluid chambers. While functionally effective, this structure is not cost effective from a manufacturing standpoint.